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Internet Engineering Task Force (IETF) M. Baeuerle
Request for Comments: 8315 STZ Elektronik
Updates: 5537 February 2018
Category: Standards Track
ISSN: 2070-1721
Cancel-Locks in Netnews Articles
Abstract
This document defines an extension to the Netnews Article Format that
may be used to authenticate the withdrawal of existing articles.
This document updates RFC 5537.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8315.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Baeuerle Standards Track [Page 1]
RFC 8315 Cancel-Locks February 2018
Table of Contents
1. Introduction ....................................................2
1.1. Conventions Used in This Document ..........................3
2. Header Fields ...................................................3
2.1. Cancel-Lock ................................................4
2.2. Cancel-Key .................................................4
3. Use .............................................................5
3.1. Adding an Initial Cancel-Lock Header Field to a
Proto-Article ..............................................5
3.2. Extending the Cancel-Lock Header Field of a Proto-Article ..6
3.3. Adding a Cancel-Key Header Field to a Proto-Article ........6
3.4. Extending the Cancel-Key Header Field of a Proto-Article ...7
3.5. Check a Cancel-Key Header Field ............................7
4. Calculating the Key Data ........................................8
5. Examples ........................................................9
5.1. Without UID ................................................9
5.2. With UID ..................................................10
5.3. Other Examples ............................................11
5.4. Manual Checks .............................................12
6. Obsolete Syntax ................................................12
7. Security Considerations ........................................13
8. IANA Considerations ............................................15
8.1. Algorithm Name Registration Procedure .....................16
8.2. Change Control ............................................16
8.3. Registration of the Netnews Cancel-Lock Hash Algorithms ...17
9. References .....................................................18
9.1. Normative References ......................................18
9.2. Informative References ....................................19
Acknowledgements ..................................................20
Author's Address ..................................................20
1. Introduction
The authentication system defined in this document is intended to be
used as a simple method to verify that the withdrawal of an article
is valid; that is to say the poster, posting agent, moderator, or
injecting agent that processed the original article has requested to
withdraw it via the use of a cancel control article
([RFC5537] Section 5.3) or a Supersedes header field
([RFC5537] Section 5.4).
This document defines two new header fields: Cancel-Lock and
Cancel-Key. The Cancel-Lock header field contains hashes of secret
data. The preimages can later be used in the Cancel-Key header field
to authenticate a cancel or supersede request.
Baeuerle Standards Track [Page 2]
RFC 8315 Cancel-Locks February 2018
One property of this system is that it prevents tracking of
individual users.
There are other authentication systems available with different
properties. When everybody should be able to verify who the
originator is, e.g., for control articles to add or remove newsgroups
([RFC5537] Section 5.2), an OpenPGP [RFC4880] signature is
appropriate.
1.1. Conventions Used in This Document
Any term not defined in this document has the same meaning as it does
in [RFC5536] or [RFC5537].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Header Fields
This section describes the formal syntax of the new header fields
using ABNF [RFC5234]. Non-terminals not defined in this document are
defined in Section 3 of [RFC5536].
The new header fields Cancel-Lock and Cancel-Key are defined by this
document, extending the list of article header fields defined in
[RFC5536].
Each of these header fields MUST NOT occur more than once in an
article.
Both new header field bodies contain lists of encoded values. Every
entry is based on a <scheme>:
scheme = "sha256" / "sha512" / 1*scheme-char / obs-scheme
scheme-char = ALPHA / DIGIT / "-" / "/"
The hash algorithms for <scheme> are defined in [RFC6234]; see also
[RFC1321] and [RFC6151] for MD5, [RFC3174] for SHA1, and [SHA] for
the SHA2 family. The Base64 encoding used is defined in Section 4 of
[RFC4648].
This document defines two values for <scheme>: "sha256" and "sha512".
The hash algorithm "sha256" is mandatory to implement.
Baeuerle Standards Track [Page 3]
RFC 8315 Cancel-Locks February 2018
Because the hash algorithm for <scheme> cannot be negotiated,
unnecessary proliferation of hash algorithms should be avoided. The
hash algorithms "sha224" and "sha384" are only added to the "Netnews
Cancel-Lock Hash Algorithms" registry (Section 8.3) because
implementations exist that support them. Implementations SHOULD NOT
use the hash algorithms "sha224" and "sha384" to generate <scheme>.
2.1. Cancel-Lock
cancel-lock = "Cancel-Lock:" SP c-lock-list CRLF
c-lock-list = [CFWS] c-lock *(CFWS c-lock) [CFWS]
c-lock = scheme ":" c-lock-string
c-lock-string = *(4base64-char) [base64-terminal]
base64-char = ALPHA / DIGIT / "+" / "/"
base64-terminal = 2base64-char "==" / 3base64-char "="
Comments in CFWS (comments and/or folding whitespace) can cause
interoperability problems, so comments SHOULD NOT be generated but
MUST be accepted.
If <scheme> is not supported by an implementation, the corresponding
<c-lock> element MUST be skipped and potential following <c-lock>
elements MUST NOT be ignored.
<c-lock-string> is the Base64-encoded output of a hash operation
(defined by <scheme>) of the Base64-encoded key "K" that is intended
to authenticate the person or agent that created or processed
(respectively) the proto-article up to injection (inclusively):
Base64(hash(Base64(K)))
Because of the one-way nature of the hash operation, the key "K" is
not revealed.
2.2. Cancel-Key
cancel-key = "Cancel-Key:" SP c-key-list CRLF
c-key-list = [CFWS] c-key *(CFWS c-key) [CFWS]
c-key = scheme ":" c-key-string
c-key-string = c-lock-string / obs-c-key-string
Comments in CFWS can cause interoperability problems, so comments
SHOULD NOT be generated but MUST be accepted.
If <scheme> is not supported by an implementation, the corresponding
<c-key> element MUST be skipped and potential following <c-key>
elements MUST NOT be ignored.
Baeuerle Standards Track [Page 4]
RFC 8315 Cancel-Locks February 2018
<c-key-string> is the Base64-encoded key "K" that was used to create
the <c-lock> element in the Cancel-Lock header field body (as defined
in Section 2.1 of this document) of the original article:
Base64(K)
The relaxed syntax definition of <c-key-string> above is required for
backward compatibility with implementations that are not compliant
with this specification. Compliant implementations SHOULD generate
valid Base64 (that is to say the syntax of <c-lock-string> as defined
in Section 2.1 of this document) and MUST accept strings of
<base64-octet> characters (that is to say the syntax of
<obs-c-key-string> as defined in Section 6 of this document).
3. Use
Use cases:
o The poster of an article wants to cancel or supersede existing
articles.
o A moderator wants the ability to cancel articles after approving
them.
o An injecting agent wants to act as a representative for a posting
agent that has no support for the authentication system described
in this document.
o A news administrator wants the ability to cancel articles that
were injected by its system (because, for example, they violate
its abuse policy).
3.1. Adding an Initial Cancel-Lock Header Field to a Proto-Article
A Cancel-Lock header field MAY be added to a proto-article by the
poster or posting agent and will include one or more <c-lock>
elements.
If the poster or posting agent doesn't add a Cancel-Lock header field
to a proto-article, then an injecting agent (or moderator) MAY add
one, including one or more <c-lock> elements.
If multiple <c-lock> elements are added to the Cancel-Lock header
field by a single agent, each <c-lock> element MUST use a unique
key "K" to improve security.
Baeuerle Standards Track [Page 5]
RFC 8315 Cancel-Locks February 2018
If an injecting agent (or moderator) wants to act as a representative
for a posting agent without support for the authentication system
described in this document, then it MUST be able to positively
authenticate the poster and MUST be able to automatically add a
working Cancel-Key header field for all proto-articles with
cancelling or superseding attempts from that poster.
Other agents MUST NOT add this header field to articles or
proto-articles that they process.
3.2. Extending the Cancel-Lock Header Field of a Proto-Article
If a Cancel-Lock header field has already been added to a
proto-article, then any agent further processing the proto-article up
to the injecting agent (inclusively) MAY append additional <c-lock>
elements to those already in the header field body.
If multiple <c-lock> elements are appended to the Cancel-Lock header
field by a single agent, each <c-lock> element MUST use a unique
key "K" to improve security.
If an injecting agent (or moderator) wants to act as a representative
for a posting agent without support for the authentication system
described in this document, then the same requirements apply as those
mentioned in Section 3.1.
Once an article is injected, then this header field MUST NOT be
altered. In particular, relaying agents beyond the injecting agent
MUST NOT alter it.
3.3. Adding a Cancel-Key Header Field to a Proto-Article
The Cancel-Key header field contains one or more of the secret
strings that were used to create the Cancel-Lock header field of the
original article. Knowledge of at least one of the secret strings is
required to create a match for successful authentication.
A Cancel-Key header field MAY be added to a proto-article containing
a Control or Supersedes header field by the poster or posting agent
and will include one or more <c-key> elements. They will correspond
to some or all of the <c-lock> elements in the article referenced by
the Control (with a "cancel" command as defined in [RFC5537]) or
Supersedes header field.
Baeuerle Standards Track [Page 6]
RFC 8315 Cancel-Locks February 2018
If, as mentioned in Section 3.1, an injecting agent or moderator
(acting as a representative for the posting agent) has added a
Cancel-Lock header field to an article listed in the Control (with a
"cancel" command as defined in [RFC5537]) or Supersedes header field,
then (given that it authenticates the poster as being the same as the
poster of the original article) it MUST add the Cancel-Key header
field with at least one <c-key> element that corresponds to that
article.
Other agents MUST NOT alter this header field.
3.4. Extending the Cancel-Key Header Field of a Proto-Article
If a Cancel-Key header field has already been added to a
proto-article, then any agent further processing the proto-article
up to the injecting agent (inclusively) MAY append additional <c-key>
elements to those already in the header field body.
If, as mentioned in Section 3.2, an injecting agent or moderator
(acting as a representative for the posting agent) has extended the
Cancel-Lock header field in an article listed in the Control (with a
"cancel" command as defined in [RFC5537]) or Supersedes header field,
then (given that it authenticates the poster as being the same as the
poster of the original article) it MUST extend the Cancel-Key header
field body with at least one <c-key> element that corresponds to that
article.
Once an article is injected, then this header field MUST NOT be
altered. In particular, relaying agents beyond the injecting agent
MUST NOT alter it.
3.5. Check a Cancel-Key Header Field
When a relaying or serving agent receives an article that attempts to
cancel or supersede a previous article via a Control (with a "cancel"
command as defined in [RFC5537]) or Supersedes header field, the
system defined in this document can be used for authentication. The
general handling of articles containing such attempts as defined in
[RFC5537] is not changed by this document.
To process the authentication, the received article must contain a
Cancel-Key header field and the original article must contain a
Cancel-Lock header field. If this is not the case, the
authentication is not possible (failed).
Baeuerle Standards Track [Page 7]
RFC 8315 Cancel-Locks February 2018
For the authentication check, every supported <c-key> element from
the received article is processed as follows:
1. The <c-key-string> part of the <c-key> element is hashed using
the algorithm defined by its <scheme> part.
2. For each <c-lock> element with the same <scheme> in the original
article, its <c-lock-string> part is compared to the calculated
hash.
3. If a <c-lock-string> part is equal to the calculated hash, the
authentication is passed and the processing of further elements
can be aborted.
4. If no match was found and there are no more <c-key> elements to
process, the authentication failed.
4. Calculating the Key Data
The following algorithm is RECOMMENDED to calculate the key "K" based
on a local secret <sec>.
The result of the function
K = HMAC(sec, uid+mid)
is the key "K" for an article with a Message-ID <mid> that belongs to
the User-ID (or UID) <uid> (e.g., the login name of the user). The
Hashed Message Authentication Code (HMAC) is outlined in [RFC2104].
The HMAC is computed over the data <uid+mid> (with "+" representing
the concatenation operation), using <sec> as a secret key held
locally that can be used for multiple articles. This method removes
the need for a per-article database containing the keys used for
every article.
A posting agent must add the Message-ID header field to the
proto-article itself and use the content of the header field body as
<mid> (excluding whitespace but including literal angle brackets).
The User-ID <uid> must not contain angle brackets (to ensure that
concatenation of different <uid> and <mid> elements cannot give the
same results).
A posting agent that uses a dedicated local secret <sec> for every
user should use an empty string for the <uid> part.
In general, different values for the secret <sec> must be used if
multiple <c-lock> elements are added by a single agent.
Baeuerle Standards Track [Page 8]
RFC 8315 Cancel-Locks February 2018
The local secret <sec> should have a length of at least the output
size of the hash function that is used by the HMAC
(256 bits / 32 octets for SHA256) and must be a cryptographically
random value [RFC4086].
Note that the hash algorithm used as the base for the HMAC operation
is not required to be the same as that specified by <scheme>. An
agent that verifies a Cancel-Key header field body simply checks
whether one of its <c-key> elements matches one of the <c-lock>
elements with the same <scheme> in the Cancel-Lock header field body
of the original article.
Common libraries like OpenSSL can be used for the cryptographic
operations.
5. Examples
5.1. Without UID
Example data for creation of a <c-lock> element with HMAC-SHA256 and
an empty string as <uid> (as recommended in Section 4 for posting
agents):
Message-ID: <12345@mid.example>
mid: <12345@mid.example>
sec: ExampleSecret
K : HMAC-SHA256(sec, mid) ;mid used as data, sec as secret key
Calculation of Base64(K) using the OpenSSL command-line tools in a
POSIX shell:
$ printf "%s" "<12345@mid.example>" \
| openssl dgst -sha256 -hmac "ExampleSecret" -binary \
| openssl enc -base64
qv1VXHYiCGjkX/N1nhfYKcAeUn8bCVhrWhoKuBSnpMA=
This can be used as <c-key-string> for cancelling or superseding the
article <12345@mid.example>.
Baeuerle Standards Track [Page 9]
RFC 8315 Cancel-Locks February 2018
Calculation of Base64(SHA256(Base64(K))) required for <c-lock-string>
using the OpenSSL command-line tools in a POSIX shell:
$ printf "%s" "qv1VXHYiCGjkX/N1nhfYKcAeUn8bCVhrWhoKuBSnpMA=" \
| openssl dgst -sha256 -binary \
| openssl enc -base64
s/pmK/3grrz++29ce2/mQydzJuc7iqHn1nqcJiQTPMc=
Inserted into the Cancel-Lock header field body of the article
<12345@mid.example>, it looks like this:
Cancel-Lock: sha256:s/pmK/3grrz++29ce2/mQydzJuc7iqHn1nqcJiQTPMc=
Inserted into the Cancel-Key header field body of an article that
should cancel or supersede the article <12345@mid.example>, it looks
like this:
Cancel-Key: sha256:qv1VXHYiCGjkX/N1nhfYKcAeUn8bCVhrWhoKuBSnpMA=
5.2. With UID
Example data for creation of a <c-lock> element with HMAC-SHA256 and
"JaneDoe" as <uid> (as recommended in Section 4):
Message-ID: <12345@mid.example>
uid: JaneDoe
mid: <12345@mid.example>
sec: AnotherSecret
K : HMAC-SHA256(sec, uid+mid) ;uid+mid as data, sec as secret key
Calculation of Base64(K) using the OpenSSL command-line tools in a
POSIX shell:
$ printf "%s" "JaneDoe<12345@mid.example>" \
| openssl dgst -sha256 -hmac "AnotherSecret" -binary \
| openssl enc -base64
yM0ep490Fzt83CLYYAytm3S2HasHhYG4LAeAlmuSEys=
This can be used as <c-key-string> for cancelling or superseding the
article <12345@mid.example>.
Baeuerle Standards Track [Page 10]
RFC 8315 Cancel-Locks February 2018
Calculation of Base64(SHA256(Base64(K))) required for <c-lock-string>
using the OpenSSL command-line tools in a POSIX shell:
$ printf "%s" "yM0ep490Fzt83CLYYAytm3S2HasHhYG4LAeAlmuSEys=" \
| openssl dgst -sha256 -binary \
| openssl enc -base64
NSBTz7BfcQFTCen+U4lQ0VS8VIlZao2b8mxD/xJaaeE=
Inserted into the Cancel-Lock header field body of the article
<12345@mid.example>, it looks like this:
Cancel-Lock: sha256:NSBTz7BfcQFTCen+U4lQ0VS8VIlZao2b8mxD/xJaaeE=
Inserted into the Cancel-Key header field body of an article that
should cancel or supersede the article <12345@mid.example>, it looks
like this:
Cancel-Key: sha256:yM0ep490Fzt83CLYYAytm3S2HasHhYG4LAeAlmuSEys=
5.3. Other Examples
Another matching pair of Cancel-Lock and Cancel-Key header fields:
Cancel-Lock: sha256:RrKLp7YCQc9T8HmgSbxwIDlnCDWsgy1awqtiDuhedRo=
Cancel-Key: sha256:sSkDke97Dh78/d+Diu1i3dQ2Fp/EMK3xE2GfEqZlvK8=
With obsolete syntax (uses a <c-key-string> with invalid/missing
Base64 padding):
Cancel-Lock: sha1:bNXHc6ohSmeHaRHHW56BIWZJt+4=
Cancel-Key: ShA1:aaaBBBcccDDDeeeFFF
Let's assume that all the examples above are associated to the same
article (e.g., created by different agents):
Cancel-Lock: sha256:s/pmK/3grrz++29ce2/mQydzJuc7iqHn1nqcJiQTPMc=
sha256:NSBTz7BfcQFTCen+U4lQ0VS8VIlZao2b8mxD/xJaaeE=
sha256:RrKLp7YCQc9T8HmgSbxwIDlnCDWsgy1awqtiDuhedRo=
sha1:bNXHc6ohSmeHaRHHW56BIWZJt+4=
Cancel-Key: sha256:qv1VXHYiCGjkX/N1nhfYKcAeUn8bCVhrWhoKuBSnpMA=
sha256:yM0ep490Fzt83CLYYAytm3S2HasHhYG4LAeAlmuSEys=
sha256:sSkDke97Dh78/d+Diu1i3dQ2Fp/EMK3xE2GfEqZlvK8=
ShA1:aaaBBBcccDDDeeeFFF
Remember that parsing for <scheme> must be case insensitive.
Baeuerle Standards Track [Page 11]
RFC 8315 Cancel-Locks February 2018
5.4. Manual Checks
Manual checks using the OpenSSL command-line tools in a POSIX shell:
$ printf "%s" "qv1VXHYiCGjkX/N1nhfYKcAeUn8bCVhrWhoKuBSnpMA=" \
| openssl dgst -sha256 -binary \
| openssl enc -base64
s/pmK/3grrz++29ce2/mQydzJuc7iqHn1nqcJiQTPMc=
$ printf "%s" "yM0ep490Fzt83CLYYAytm3S2HasHhYG4LAeAlmuSEys=" \
| openssl dgst -sha256 -binary \
| openssl enc -base64
NSBTz7BfcQFTCen+U4lQ0VS8VIlZao2b8mxD/xJaaeE=
$ printf "%s" "sSkDke97Dh78/d+Diu1i3dQ2Fp/EMK3xE2GfEqZlvK8=" \
| openssl dgst -sha256 -binary \
| openssl enc -base64
RrKLp7YCQc9T8HmgSbxwIDlnCDWsgy1awqtiDuhedRo=
$ printf "%s" "aaaBBBcccDDDeeeFFF" \
| openssl dgst -sha1 -binary \
| openssl enc -base64
bNXHc6ohSmeHaRHHW56BIWZJt+4=
6. Obsolete Syntax
Implementations of earlier draft versions of this specification
defined a different value for <scheme> than this version. The
following value for <scheme> is now deprecated and SHOULD NOT be
generated anymore. Serving agents SHOULD still accept it for a
transition period as long as the corresponding hash function is not
considered unsafe (see Section 7 for details) or already marked as
OBSOLETE in the "Netnews Cancel-Lock Hash Algorithms" registry
(Section 8.3).
obs-scheme = "sha1"
It is important for backward compatibility that the deprecated value
for <scheme> is not phased out too early. Security and compatibility
concerns should be carefully weighed before choosing to remove
<obs-scheme> from existing implementations (or not implementing it in
new ones).
Baeuerle Standards Track [Page 12]
RFC 8315 Cancel-Locks February 2018
Earlier draft versions of this specification allowed more liberal
syntax for <c-key-string>:
obs-c-key-string = 1*base64-octet
base64-octet = ALPHA / DIGIT / "+" / "/" / "="
<obs-c-key-string> SHOULD NOT be generated but MUST be accepted.
7. Security Considerations
The authentication system defined in this document provides no
integrity-checking properties. Arbitrary modifications can be
applied to an article on its way through the network, regardless of
the presence of a Cancel-Key header field. A serving agent that
receives an article that contains a Cancel-Key header field with a
matching <c-key> element only gets the information that the
withdrawal of the target article was approved by a legitimate person
or agent.
Example: A valid <c-key> element is extracted from a cancel control
article and inserted into a forged supersede article. All servers on
the network that receive the forged supersede article before the
cancel control article should accept the forged supersede. But
because everybody can post articles with forged identity information
in the header (same as with spam email), the same result can be
achieved by sending a forged new article using no authentication
system at all.
For originator and integrity checks, a signature-based authentication
system is required (normally, OpenPGP [RFC4880] is used for this
purpose). Both systems can be combined.
The important property of the hash function used for <scheme> is the
preimage resistance. A successful preimage attack either reveals the
real Cancel-Key (that was used to create the Cancel-Lock of the
original article) or gives a different Cancel-Key (that matches a
Cancel-Lock too). This would break the authentication system defined
in this document.
Collision resistance of the hash function used for <scheme> is less
important. Finding two <c-key> elements for the Cancel-Key header
field that match to a <c-lock> element of an arbitrary Cancel-Lock
header field is not helpful to break the authentication system
defined in this document (if a specific article is defined as the
target). Only collateral damage by arbitrary cancel or supersede is
possible.
Baeuerle Standards Track [Page 13]
RFC 8315 Cancel-Locks February 2018
Currently, there is no known practicable preimage and second preimage
attack against the hash function SHA1. Therefore, there is no hurry
to replace it. The reasons why this document specifies hash
functions from the SHA2 family are:
o The previous specification of the authentication system defined in
this document -- [USEFOR-CANCEL-LOCK] -- is nearly two decades
old. The client-side implementations are moving forward extremely
slowly, too (newsreaders from the last millennium are still in
heavy use). What is defined today should be strong enough for the
next two decades or so.
o The collision resistance of SHA1 is already broken; therefore, it
is now obsolete for digital signatures as used in Transport Layer
Security (TLS). It is intended that an implementation of the
authentication system defined in this document can share the same
cryptographic library functions that are used for TLS.
o It is intended that the same hash function can be used for
<scheme> and (as the base) for the HMAC that is recommended in
Section 4. See notes below for HMAC-MD5 and HMAC-SHA1.
o The SHA2 family of hash algorithms is widely supported by
cryptographic libraries. In contrast, SHA3 is currently too
recent and has not been studied enough to be considered more
secure than SHA2.
The operation HMAC(sec, uid+mid) as recommended in Section 4 must be
able to protect the local secret <sec>. The Message-ID <mid> is
public (in the Message-ID header field body), and <uid> is optional.
An attacker who wants to steal/use a local secret only needs to break
this algorithm (regardless of <scheme>), because Cancel-Key header
fields are explicitly published for every request to cancel or
supersede existing articles.
Even if HMAC-MD5 and HMAC-SHA1 are not considered broken today, it is
desired to have a greater margin for security here. Breaking
<scheme> only allows the authentication of a single forged cancel or
supersede request. With <sec> in hand, it is possible to forge such
requests for all articles that contain Cancel-Lock header field
bodies with elements that were generated with this <sec> in the past.
Changing <sec> at regular intervals can be used to mitigate potential
damage.
Baeuerle Standards Track [Page 14]
RFC 8315 Cancel-Locks February 2018
If an agent adds or appends multiple <c-lock> elements, it must not
use the same K for them (by using different secrets (<sec>)). Adding
multiple <c-lock> elements with the same <scheme> and the same K
makes no sense (because it would result in identical <c-lock>
elements); therefore, the case of different <scheme> values is
relevant: a preimage attack on the different hash algorithms may be
easier if the attacker knows that the output of those hash algorithms
was created with the same input.
If an implementation chooses to not implement the key calculation
algorithm recommended in Section 4 or to implement it with the HMAC
based on a different hash function than <scheme>, the key size used
should match the output size of the hash function used for <scheme>.
8. IANA Considerations
IANA has registered the following header fields in the "Permanent
Message Header Field Names" registry, in accordance with the
procedures set out in [RFC3864]:
Header field name: Cancel-Lock
Applicable protocol: netnews
Status: standard
Author/change controller: IETF
Specification document(s): RFC 8315
Header field name: Cancel-Key
Applicable protocol: netnews
Status: standard
Author/change controller: IETF
Specification document(s): RFC 8315
The "Netnews Cancel-Lock Hash Algorithms" registry is maintained by
IANA.
The registry is available at
<https://www.iana.org/assignments/netnews-parameters/>.
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RFC 8315 Cancel-Locks February 2018
8.1. Algorithm Name Registration Procedure
IANA will register new Cancel-Lock hash algorithm names on a First
Come First Served basis, as defined in BCP 26 [RFC8126]. IANA has
the right to reject obviously bogus registration requests but will
perform no review of claims made in the registration form.
Registration of a Netnews Cancel-Lock hash algorithm is requested by
filling in the following template and sending it via electronic mail
to IANA at <iana@iana.org>:
Subject: Registration of Netnews Cancel-Lock hash algorithm X
Netnews Cancel-Lock hash algorithm name:
Security considerations:
Published specification (recommended):
Contact for further information:
Intended usage: (One of COMMON, LIMITED USE, or OBSOLETE)
Owner/Change controller:
Note: (Any other information that the author deems relevant may be
added here.)
Any name that conforms to the syntax of a Netnews Cancel-Lock hash
algorithm (see the definition of <scheme> in Section 2) can be used;
in particular, Netnews Cancel-Lock algorithms are named by strings
consisting of letters, digits, hyphens, and/or slashes.
Authors may seek community review by posting a specification of their
proposed algorithm as an Internet-Draft. Netnews Cancel-Lock hash
algorithms intended for widespread use should be standardized through
the normal IETF process, when appropriate.
The IESG is considered to be the owner of all Netnews Cancel-Lock
hash algorithms that are on the IETF Standards Track.
8.2. Change Control
Once a Netnews Cancel-Lock hash algorithm registration has been
published by IANA, the owner may request a change to its definition.
The change request follows the same procedure as the initial
registration request.
The owner of a Netnews Cancel-Lock hash algorithm may pass
responsibility for the algorithm to another person or agency by
informing IANA; this can be done without discussion or review.
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RFC 8315 Cancel-Locks February 2018
The IESG may reassign responsibility for a Netnews Cancel-Lock hash
algorithm. The most common reason for this would be to enable
changes to be made to algorithms where the owner of the registration
has died, has moved out of contact, or is otherwise unable to make
changes that are important to the community.
Netnews Cancel-Lock hash algorithm registrations MUST NOT be deleted.
Algorithms that are no longer believed appropriate for use can be
declared OBSOLETE by a change to their "intended usage" field; such
algorithms will be clearly marked in the registry published by IANA.
The IESG is considered to be the owner of all Netnews Cancel-Lock
hash algorithms that are on the IETF Standards Track.
8.3. Registration of the Netnews Cancel-Lock Hash Algorithms
This section gives a formal definition of the Netnews Cancel-Lock
hash algorithms as required by Section 8.1 for the IANA registry.
Netnews Cancel-Lock hash algorithm name: md5
Security considerations: See Section 7 of this document
Published specification: RFC 8315
Contact for further information: Author of this document
Intended usage: OBSOLETE
Owner/Change controller: IESG <iesg@ietf.org>
Note: Do not use this algorithm anymore
Netnews Cancel-Lock hash algorithm name: sha1
Security considerations: See Section 7 of this document
Published specification: RFC 8315
Contact for further information: Author of this document
Intended usage: LIMITED USE
Owner/Change controller: IESG <iesg@ietf.org>
Note: This algorithm is intended for backward compatibility
Netnews Cancel-Lock hash algorithm name: sha224
Security considerations: See Section 7 of this document
Published specification: RFC 8315
Contact for further information: Author of this document
Intended usage: LIMITED USE
Owner/Change controller: IESG <iesg@ietf.org>
Note: sha256 should be used instead; this is a truncated variant
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RFC 8315 Cancel-Locks February 2018
Netnews Cancel-Lock hash algorithm name: sha256
Security considerations: See Section 7 this document
Published specification: RFC 8315
Contact for further information: Author of this document
Intended usage: COMMON
Owner/Change controller: IESG <iesg@ietf.org>
Note: This algorithm is mandatory to implement
Netnews Cancel-Lock hash algorithm name: sha384
Security considerations: See Section 7 of this document
Published specification: RFC 8315
Contact for further information: Author of this document
Intended usage: LIMITED USE
Owner/Change controller: IESG <iesg@ietf.org>
Note: sha512 should be used instead; this is a truncated variant
Netnews Cancel-Lock hash algorithm name: sha512
Security considerations: See Section 7 of this document
Published specification: RFC 8315
Contact for further information: Author of this document
Intended usage: COMMON
Owner/Change controller: IESG <iesg@ietf.org>
Note: This algorithm is optional
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
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RFC 8315 Cancel-Locks February 2018
[RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5536] Murchison, K., Ed., Lindsey, C., and D. Kohn, "Netnews
Article Format", RFC 5536, DOI 10.17487/RFC5536,
November 2009, <https://www.rfc-editor.org/info/rfc5536>.
[RFC5537] Allbery, R., Ed., and C. Lindsey, "Netnews Architecture
and Protocols", RFC 5537, DOI 10.17487/RFC5537,
November 2009, <https://www.rfc-editor.org/info/rfc5537>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/info/rfc6234>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in
RFC 2119 Key Words", BCP 14, RFC 8174,
DOI 10.17487/RFC8174, May 2017,
<https://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992,
<https://www.rfc-editor.org/info/rfc1321>.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.
[RFC3174] Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
(SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,
<https://www.rfc-editor.org/info/rfc3174>.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880,
DOI 10.17487/RFC4880, November 2007,
<https://www.rfc-editor.org/info/rfc4880>.
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RFC 8315 Cancel-Locks February 2018
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, DOI 10.17487/RFC6151, March 2011,
<https://www.rfc-editor.org/info/rfc6151>.
[SHA] National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", FIPS 180-4,
DOI 10.6028/NIST.FIPS.180-4, August 2015,
<http://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.180-4.pdf>.
[USEFOR-CANCEL-LOCK]
Lyall, S., "Cancel-Locks in Usenet articles.", Work in
Progress, draft-ietf-usefor-cancel-lock-01, November 1998.
Acknowledgements
The author acknowledges the original author of the Cancel-Lock
authentication system, as documented in [USEFOR-CANCEL-LOCK]: Simon
Lyall. Simon wrote the original document and approved the usage of
his work for this document. This document is mostly based on his
work. (It was originally intended as revision 02 but was renamed
because the USEFOR IETF WG is now closed.)
The author would like to thank the following individuals for
contributing their ideas and reviewing this specification: Russ
Allbery, Urs Janssen, Richard Kettlewell, Marcel Logen, Holger
Marzen, Dennis Preiser, and Emil Schuster. Thanks also to Peter
Faust and Alfred Peters for providing statistical data about the
algorithms currently in use.
Special thanks to the Document Shepherd, Julien Elie; and to the
responsible Area Director, Alexey Melnikov.
Author's Address
Michael Baeuerle
STZ Elektronik
Hofener Weg 33C
Remseck, Baden-Wuerttemberg 71686
Germany
Fax: +49 7146 999061
Email: michael.baeuerle@stz-e.de
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